Field of the Invention
The present invention relates to a carrier having low specific gravity in which the content of volatile organic compounds (VOC) is suppressed to a fixed range, and specifically relates to a carrier used in a two-component electrophotographic developer used for copying machines, printers, and the like, and an electrophotographic developer using the carrier.
Description of the Related Art
An electrophotographic developing method is a method of developing by adhering toner particles in a developer to an electrostatic latent image formed on a photo conductor, and the developer used in this method is divided into a two-component developer including toner particles and carrier particles and a one-component developer using only toner particles.
Among such developers, a cascade process and the like used to be employed as a developing method using the two-component developer including toner particles and carrier particles, but a magnetic brush method for using a magnet roll is mainly used at present.
In the two-component developer, the carrier particles are a carrier material for imparting desired charge to the toner particles by being stirred with the toner particles in a developing box filled with the developer and conveying the toner particles carrying charge in this way to the surface of the photo conductor to form a toner image on the photo conductor. The carrier particles remaining on a developing roll holding a magnet return from the developing roll to the developing box again, are mixed and stirred with new toner particles, and are repeatedly used for a certain period.
Unlike the one-component developer, in the two-component developer, the carrier particles have a function in which the carrier particles are mixed and stirred with the toner particles to triboelectrically charge and convey the toner particles. Thus, the two-component developer has good controllability in designing a developer. Therefore, the two-component developer is suitable for a full color development apparatus requiring high image quality, an apparatus performing high-speed printing requiring reliability and durability of image maintenance, and the like.
In the two-component developer used in this way, it is required that image performance such as image density, fogging, white spots, tone reproduction, and resolution show a specific value from the initial stage, and this performance does not fluctuate during endurance printing and is stably maintained. In order to stably maintain this performance, it is necessary that the performance of carrier particles contained in the two-component developer be stable.
Various kinds of carriers such as an iron powder carrier, a ferrite carrier, a resin-coated ferrite carrier, and a magnetic powder-dispersed resin carrier have conventionally been used as the carrier particles forming the two-component developer.
Recently, an office network has advanced; a copying machine has evolved from a single function copying machine to the age of a composite machine; and a service system has also shifted from a system in which contracted service technicians periodically perform maintenance to replace a developer and the like to the age of a maintenance free system. Thus, a request from the market for further extension of life of a developer has been increasing.
Under such circumstances, attempts have been made to achieve extension of life of a carrier, eventually, a developer by achieving decrease in specific gravity of a carrier to reduce the stress by stirring to prevent shaving and peeling of a layer.
In Japanese Patent Laid-Open No. 2007-034249, a core material having low specific gravity is produced by forming a hollow during sintering by adding a foaming agent in combination with a magnetic component during granulation. However, since a large void is present in particles, strength is low, and when the core material is used as a developer, a crack and a chip occur to cause reduction in charging ability and carrier scattering. Further, a core material having low specific gravity is produced by forming a composite by adding silica during granulation. However, since Si reacts with a magnetic component during sintering, magnetization required for a core material cannot be satisfied, and there is apprehension that charge performance is rendered unsatisfactory by the presence of silica in the core material, and that the strength is reduced.
Further, in Japanese Patent Laid-Open No. 2009-244572, a granulated material having low specific gravity is used as a raw material, which is thermally sprayed to thereby produce a hollow core material to decrease specific gravity. However, since all particles do not have a hollow structure, these particles are apparently light but heavy particles are mixed therewith, and when these particles are mixed and stirred with toner particles, toner fuses with a carrier to produce toner spent.
In Japanese Patent Laid-Open No. 2012-215858, decrease in specific gravity has been achieved by coating porous ferrite particles with resin. However, since this is a production method in which a part of the resin can penetrate the voids of ferrite particles, coat thickness easily varies according to the state of penetration, and it is difficult to control charge performance. Further, when compared with a non-porous core material, the coating amount of the resin will be adjusted by considering the amount of penetration of the resin into a porous core material, and an economic disadvantage will occur.
In Japanese Patent Laid-Open No. 2013-250455, a resin precursor solution in which magnetic particles are dispersed is used to form resin particles in which magnetic particles are dispersed in the process of polymerization, thereby producing a so-called resin carrier having low specific gravity. However, such problems occur that image density is not obtained since carrier resistivity is high; magnetic particulates are eliminated and damage a photo conductor; charge rising performance is poor since residual magnetization and coercive force are high; and the like.
Japanese Patent Laid-Open No. 2014-197040 describes a ferrite carrier core material including porous ferrite particles and a resin-filled ferrite carrier in which voids of the core material are filled with resin. These techniques have allowed production of a carrier that satisfies all the problems accompanying decrease in specific gravity, such as strength, magnetic force, charge, resistivity and the like.
As described in these Patent Literatures, many attempts to decrease specific gravity of a carrier have been performed, and a part of the attempts has been achieved.
On the other hand, reduction of the environmental load is required also for a carrier, and therefore, it is preferred that heavy metals such as Cu, Zn, and Ni be not contained at a higher level than the range of unavoidable impurities (associated impurities) in the carrier composition.
Further, from the point of view of reduction of the environmental load, it is similarly requested that volatile organic compounds (VOC), particularly highly toxic aldehydes, in a carrier be reduced.
However, the cited Japanese Patent Laid-Open No. 2013-250455 has a problem that since an aldehyde is used in the production process, the aldehyde remains in a carrier as a volatile organic compound. The cited Japanese Patent Laid-Open No. 2014-197040 has a problem that since an organic solvent such as toluene is used in the production step, a volatile organic compound such as toluene remains in a carrier.
Therefore, an object of the present invention is to provide a carrier in which decrease in specific gravity is achieved and a volatile organic compound (VOC) is reduced and to provide an electrophotographic developer using the carrier.